1. Field of the Invention
The present invention relates to a reception device for microwave signals. The application of the invention lies in the production of point-to-point links in the field of directional millimeter radio.
A microwave signal reception system is conventionally composed of filters, amplifiers, mixers and attenuators, as well as slaving means for setting the level of the signal at the system output. It is in actual fact necessary to control the gain of the reception system because, unlike with cable transmissions, in directional radio the received power can undergo very strong variations on account of changes in the propagation conditions. The gain of the reception system can generally be set by means of voltage-controlled variable attenuators.
The conventional way of ensuring correct operation of the reception system when it is installed is to extract information from the output of the system which is representative of the power level received at the input of the system, and to compare this information with the actual power level received by the system. The latter level is determined by the power level transmitted by the transmission system of the link, from which the losses due to attenuation in free space have been subtracted. In order for the result of the comparison to be usable, it is necessary for the information to be as representative as possible of the received level, which is why the information needs to be linearly dependent on the received level.
2. Description of the Related Art
There are a number of known solutions for obtaining information of this type. Firstly, provision may be made to use a reception system whose amplification is logarithmic. This system then delivers a signal whose voltage is perfectly linear as a function of the power level at the input of the system. However, this type of system is applicable only for constant-envelope signals which do not require amplitude linearity from the reception system. This system is therefore of limited interest.
A second possibility consists in using a reception system whose amplification is linear and in using the control voltage delivered by the slaving means (automatic gain control) of the system. By virtue of its linearity, this reception system is capable of processing any type of signals. The control voltage is dependent on the power level received at the input of the system, but the corresponding curve is of logarithmic shape, which means that the slope of the curve is high for low power levels and low for high levels. This solution is therefore not satisfactory because the information which is gathered is not then precise enough for high power levels.
A third solution might consist in supplementing the preceding solution by providing an additional unit having an exponential transfer function, in order to convert the control voltage into a linear function. However, this solution poses problems as regards the practical realization of the unit, in particular as regards its thermal stability and its reproducibility.